Usually, injectors for internal combustion engines comprise a metering servovalve having a control chamber, which communicates with a fuel inlet and with a fuel discharge channel. The metering servovalve comprises a shutter, which is axially movable under the action of an electro-actuator to open/close an outlet opening of the discharge channel and vary the pressure in the control chamber. The pressure in the control chamber, in turn, controls the opening/closing of an end nozzle of the injector to supply the fuel in a associated cylinder.
The discharge channel has a calibrated segment, which is of particular importance for correct operation of the metering servovalve. In particular, in this calibrated segment, a fluid flow rate is associated with a predefined pressure differential.
In the injectors that are produced, the calibrated segment of the discharge channel is produced by making a perforation via electron discharge machining, followed by a finishing operation, necessary to eliminate any perforation defects that, even if small, would in any case result in large pressure drop errors in the flow of fuel and, consequently, in the flow rate of fuel leaving the control chamber.
In particular, the finishing operation is of an experimental nature and is carried out by making an abrasive liquid flow through the hole made via electron discharge machining, setting the pressure upstream and downstream of the hole and detecting the flow rate: the flow rate tends to increase progressively with the abrasion caused by the liquid on the lateral surface of the hole, until a preset design value is reached. At this point, the flow is interrupted: in usage, the section of the final passage obtained shall determine, with close approximation, a pressure drop equal to the difference in pressure established upstream and downstream of the hole during the finishing operation and a flow rate of fuel leaving the control chamber equal to the preset design value.
In the injector disclosed in patent EP1612403, the discharge channel has an outlet made in an axial stem guiding the shutter, which is defined by a sliding sleeve. The calibrated segment of the discharge channel is coaxial with the axial stem and is made in a perforated plate, which axially delimits the control chamber. Downstream of this calibrated segment, the discharge channel comprises an axial segment and then two opposed radial sections, which define, together, a relatively large passage section for the discharged fuel. Considering, for example, a fuel supply pressure of approximately 1600 bar to the injector, when the metering servovalve is open, or rather when the sleeve that defines the shutter is raised in the open position, the fuel inlet that runs into the control chamber determines a pressure drop down to approximately 700 bar in the control chamber; then, between the upstream and downstream ends of the calibrated segment of the discharge channel, the fuel pressure drops from approximately 700 bar to a few bar.
The curve shown with a line in FIG. 16 is an experimental curve that qualitatively shows the pressure trend of the fuel flow leaving the control chamber when the servovalve is open. A pressure P1 (approximately equal to 700 bar, as indicated above) is present in the control chamber, while in the discharge environment, downstream of the seal between the axial stem and the sleeve that defines the shutter, pressure PSCAR is present. The linearized distance with respect to the control chamber is shown on the abscissa. In particular:                XA: position immediately next to the outlet of the calibrated segment,        XRAD: inlet position on the two opposed radial sections,        XTEN: position at the sealing zone between the axial stem and the sleeve that defines the shutter,        XSCA: position in the discharge environment in which the fuel pressure stabilizes itself.        
Experimentally, due to the large pressure drop, the onset of cavitation is encountered. In other words, the fuel pressure upstream of the discharge environment drops below the vapour pressure, indicated as PVAPOR, in correspondence to the outlet from the calibrated segment, where fuel flow velocity is maximum and the pressure is minimum (PMIN). In particular, the fraction or percentage of vapour is close to one.
As the passage sections from position XA to position XTEN are relatively narrow (even if larger than that of the calibrated segment), the fuel pressure slowly rises, and not all of the vapour that formed immediately downstream of position XA returns to the liquid state.
Thus, in correspondence to position XTEN the vapour fraction is still substantial. In correspondence to position XTEN, there is then the maximum increase in passage section. In this zone, it is possible to distinguish three undesired phenomena:                due to the rapid increase in passage section, the pressure tends to rise and the previously formed vapour bubbles tend to implode; when this phenomenon takes place next to the surfaces that define the seal, it causes undesired wear on these surfaces,        during closure of the shutter, contact between the surfaces that define the seal takes place in the presence of vapour, namely in “dry” conditions, with consequent impacts that cause further wear, and        in addition, always due to these “dry” conditions, the damping effect of the liquid is lost and shutter rebound occurs, which causes a delay in closing the servovalve, with a consequent undesired increase in the amount of injected fuel with respect to that established by design.        
Summarizing: the wear deriving from the above-stated phenomena greatly reduces injector life, while the rebounds in the closure phase make the injector inaccurate.
Moreover, to generate a pressure drop of approximately 700 bar, the calibrated segment must have an extremely small diameter, which is extremely complex to make with precision and in a constant manner across the various injectors.
The same drawbacks are present in the embodiment disclosed in the US patent application having publication number US2003/0106533, as the discharge channel substantially has the same arrangement with two opposed radial outlet segments which define, together, a relatively large passage section. Unlike the embodiment disclosed in EP1612403, the discharge channel is made in the shutter, which is defined by a axially sliding pin.